In the foodstuff field, there is a constantly increasing demand for cooked or pre-cooked foodstuffs which are proposed to consumers as frozen or non-frozen products. The foodstuffs thus prepared may be packaged in sealed bags, under vacuum, or in boxes. Thus, the cooking of the products may be advantageously carried out after the packaging thereof.
Furthermore, recent studies have enabled the optimal cooking steps for each kind of foodstuff to be defined, on purpose to keep the nutritional and gustative properties thereof, while insuring a long shelf life after their cooking.
Related studies have shown that the temperature rising, the cooling after cooking, and the stability of the proper cooking temperature were the important parameters characterizing the products. These parameters have to be precisely controlled during the whole cooking period and with regards to the proper cooking volume. Accordingly, the cooking temperature must be adjusted or stabilized to a precise level (preferably to the nearest degree) in all parts of the cooking vessel, this optimal temperature often being lower than 100.degree. C., especially about 70.degree. C.
Thus, various cooking processes have been tested, wherein baskets containing foodstuffs were sprayed with hot water at any suitable temperature. Use was also made of water-bath devices. These attempts revealed problems relating to the control of the cooking temperature and to the modifications of the temperature.
Moreover, steam cooking has many advantages, especially with regard to the dietetics, when the foodstuffs have been cooked in bulk and not as a sealed packaging. However, until recently, it was difficult and even impossible to adjust the steam cooking temperature at a temperature lower than 100.degree. C.
In FR-A- 2 625 891, there is described a device for the steam cooking of foodstuffs. This device is remarkable in that steam is gradually injected into the cooking vessel and the temperature is adjusted by controlling the pressure within said vessel. The invention relates to an improved apparatus of the above-mentioned type, which is especially intended to enable the temperature to be more efficiently controlled and the uniformity of the temperature to be improved in the useful space of the vessel.